Nitride crystal, optical device, semiconductor device, and method for manufacturing nitride crystal

ABSTRACT

According to one embodiment, a nitride crystal includes first, second, and third nitride crystal regions. The third nitride crystal region includes Al, and is provided between the first and second nitride crystal regions. A third oxygen concentration in the third nitride crystal region is greater than a first oxygen concentration in the first nitride crystal region and greater than a second oxygen concentration in the second nitride crystal region. A third carbon concentration in the third nitride crystal region is greater than a first carbon concentration in the first nitride crystal region and greater than a second carbon concentration in the second nitride crystal region. A &lt;0001&gt; direction of the first nitride crystal region is one of a first orientation from the second nitride crystal region toward the first nitride crystal region or a second orientation from the first nitride crystal region toward the second nitride crystal region.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2020-032021, filed on Feb. 27, 2020; theentire contents of which are incorporated herein by reference.

FIELD

Embodiments of the invention generally relate to a nitride crystal, anoptical device, a semiconductor device, and a method for manufacturing anitride crystal.

BACKGROUND

Nitride crystals are utilized in optical devices, semiconductor devices,etc. It is desirable to improve the characteristics of nitride crystals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a nitridecrystal according to a first embodiment;

FIG. 2 is a schematic cross-sectional view illustrating a nitridecrystal according to the first embodiment;

FIG. 3 is an electron microscope image illustrating the nitride crystalaccording to the first embodiment;

FIGS. 4A to 4D are schematic views illustrating the nitride crystalaccording to the first embodiment;

FIGS. 5A and 5B are graphs illustrating the nitride crystal according tothe first embodiment;

FIG. 6 is a schematic cross-sectional view illustrating a nitridecrystal according to the first embodiment;

FIGS. 7A to 7E are schematic cross-sectional views illustrating themethod for manufacturing the nitride crystal according to the firstembodiment;

FIG. 8 is a schematic cross-sectional view illustrating a nitridecrystal according to the first embodiment;

FIG. 9 is a schematic cross-sectional view illustrating a semiconductordevice according to a second embodiment;

FIG. 10 is a schematic cross-sectional view illustrating a semiconductordevice according to the second embodiment;

FIGS. 11A and 11B are schematic views illustrating an optical deviceaccording to a third embodiment; and

FIG. 12 is a flowchart illustrating a method for manufacturing a nitridecrystal according to a fourth embodiment.

DETAILED DESCRIPTION

According to one embodiment, a nitride crystal includes a first nitridecrystal region a second nitride crystal region, and a third nitridecrystal region. The third nitride crystal region includes Al, and isprovided between the first nitride crystal region and the second nitridecrystal region. A third oxygen concentration in the third nitridecrystal region is greater than a first oxygen concentration in the firstnitride crystal region and greater than a second oxygen concentration inthe second nitride crystal region. A third carbon concentration in thethird nitride crystal region is greater than a first carbonconcentration in the first nitride crystal region and greater than asecond carbon concentration in the second nitride crystal region. A<0001> direction of the first nitride crystal region is one of a firstorientation from the second nitride crystal region toward the firstnitride crystal region or a second orientation from the first nitridecrystal region toward the second nitride crystal region. A <0001>direction of the second nitride crystal region is the other of the firstorientation or the second orientation.

According to one embodiment, a method for manufacturing a nitridecrystal is disclosed. The method can include processing a first nitridecrystal layer in an atmosphere including oxygen. The method can include,after the processing, forming a third nitride crystal layer on the firstnitride crystal layer at a temperature of 450° C. or less with a GroupV/Group III ratio of 20000 or more. In addition, the method can includeforming a second nitride crystal layer on the third nitride crystallayer.

Various embodiments are described below with reference to theaccompanying drawings.

The drawings are schematic and conceptual; and the relationships betweenthe thickness and width of portions, the proportions of sizes amongportions, etc., are not necessarily the same as the actual values. Thedimensions and proportions may be illustrated differently amongdrawings, even for identical portions.

In the specification and drawings, components similar to those describedpreviously in an antecedent drawing are marked with like referencenumerals, and a detailed description is omitted as appropriate.

First Embodiment

FIG. 1 is a schematic cross-sectional view illustrating a nitridecrystal according to a first embodiment.

As shown in FIG. 1 , the nitride crystal 110 according to the embodimentincludes a first nitride crystal region 10, a second nitride crystalregion 20, and a third nitride crystal region 30.

The third nitride crystal region 30 is provided between the firstnitride crystal region 10 and the second nitride crystal region 20. Thethird nitride crystal region 30 includes Al. For example, the thirdnitride crystal region 30 includes AlN or AlGaN.

For example, the direction from the first nitride crystal region 10toward the second nitride crystal region 20 is taken as a Z-axisdirection. One direction perpendicular to the Z-axis direction is takenas an X-axis direction. A direction perpendicular to the Z-axisdirection and the X-axis direction is taken as a Y-axis direction.

In the example, the nitride crystal 110 includes a substrate 50 s and anitride member 55. The first nitride crystal region 10 is between thesubstrate 50 s and the second nitride crystal region 20. The firstnitride crystal region 10 is between the substrate 50 s and the thirdnitride crystal region 30. The nitride member 55 is provided between thesubstrate 50 s and the first nitride crystal region 10. In one example,the nitride member 55 includes, for example, Al and N. The nitridemember 55 may include, for example, Ga and N. The nitride member 55 is,for example, a low temperature-growth nitride semiconductor.

The concentration of oxygen (a third oxygen concentration) in the thirdnitride crystal region 30 is greater than the concentration of oxygen (afirst oxygen concentration) in the first nitride crystal region 10 andgreater than the concentration of oxygen (a second oxygen concentration)in the second nitride crystal region 20. The concentration of carbon (athird carbon concentration) in the third nitride crystal region 30 isgreater than the concentration of carbon (a first carbon concentration)in the first nitride crystal region 10 and greater than theconcentration of carbon (a second carbon concentration) in the secondnitride crystal region 20.

The <0001> direction of the first nitride crystal region 10 is one of afirst orientation from the second nitride crystal region 20 toward thefirst nitride crystal region 10 or a second orientation from the firstnitride crystal region 10 toward the second nitride crystal region 20.The <0001> direction of the second nitride crystal region 20 is theother of the first orientation or the second orientation recited above.

In the example of FIG. 1 , the <0001> direction of the first nitridecrystal region 10 is the first orientation (e.g., the −Z direction), andthe <0001> direction of the second nitride crystal region 20 is thesecond orientation (e.g., the +Z direction).

FIG. 2 is a schematic cross-sectional view illustrating a nitridecrystal according to the first embodiment.

As shown in FIG. 2 , the nitride crystal 111 according to the embodimentalso includes the first nitride crystal region 10, the second nitridecrystal region 20, and the third nitride crystal region 30. In thenitride crystal 111, the <0001> direction of the first nitride crystalregion 10 is the second orientation (e.g., the +Z direction), and the<0001> direction of the second nitride crystal region 20 is the firstorientation (e.g., the −Z direction).

When the <0001> direction of the first nitride crystal region 10 is thefirst orientation (e.g., the −Z direction) recited above, the polarityof the first nitride crystal region 10 is Group V-polar (e.g.,nitrogen-polar). When the <0001> direction of the second nitride crystalregion 20 is the second orientation (e.g., the +Z direction) recitedabove, the polarity of the second nitride crystal region 20 is GroupIII-polar (e.g., Al-polar or Ga-polar).

When the <0001> direction of the first nitride crystal region 10 is thesecond orientation (e.g., the +Z direction) recited above, the polarityof the first nitride crystal region 10 is Group III-polar (e.g.,Al-polar or Ga-polar). When the <0001> direction of the second nitridecrystal region 20 is the first orientation (e.g., the −Z direction)recited above, the polarity of the second nitride crystal is GroupV-polar (e.g., nitrogen-polar).

Thus, in the nitride crystal (the nitride crystal 110 or the nitridecrystal 111) according to the embodiment, the <0001> direction of thefirst nitride crystal region 10 is one of the first orientation recitedabove or the second orientation recited above. The <0001> direction ofthe second nitride crystal region 20 is the other of the firstorientation recited above or the second orientation recited above.

In the nitride crystal (the nitride crystal 110 or the nitride crystal111) according to the embodiment, the polarity of the first nitridecrystal region 10 is the reverse of the polarity of the second nitridecrystal region 20.

When the <0001> direction of the first nitride crystal region 10 is thefirst orientation (e.g., the −Z direction) recited above, the surface ofthe first nitride crystal region 10 on the third nitride crystal region30 side is a Group V surface (e.g., a nitrogen surface). When the <0001>direction of the second nitride crystal region 20 is the secondorientation (e.g., the +Z direction) recited above, the surface of thesecond nitride crystal region 20 on the side opposite to the thirdnitride crystal region 30 is a Group III surface (e.g., an Al surface ora Ga surface).

When the <0001> direction of the first nitride crystal region 10 is thesecond orientation (e.g., the +Z direction) recited above, the surfaceof the first nitride crystal region 10 on the third nitride crystalregion 30 side is a Group III surface (e.g., an Al surface or a Gasurface). When the <0001> direction of the second nitride crystal region20 is the first orientation (e.g., the −Z direction) recited above, thesurface of the second nitride crystal region 20 on the side opposite tothe third nitride crystal region 30 is a Group V surface (e.g., anitrogen surface).

When the surface of the first nitride crystal region 10 on the thirdnitride crystal region 30 side is a Group V surface (e.g., a nitrogensurface), the polarity of the first nitride crystal region 10 is GroupV-polar (e.g., nitrogen-polar). When the surface of the second nitridecrystal region 20 on the side opposite to the third nitride crystalregion 30 is a Group III surface (e.g., an Al surface or a Ga surface),the polarity of the second nitride crystal region 20 is Group III-polar(e.g., Al-polar or Ga-polar). A “Group V surface” (e.g., a nitrogensurface) corresponds to a “Group V-polar surface” (e.g., anitrogen-polar surface). A “Group III surface” (e.g., an Al surface or aGa surface) corresponds to a “Group III-polar surface” (e.g., anAl-polar surface or a Ga-polar surface).

When the surface of the first nitride crystal region 10 on the thirdnitride crystal region 30 side is a Group III surface (e.g., an Alsurface or a Ga surface), the polarity of the first nitride crystalregion 10 is Group III-polar (e.g., Al-polar or Ga-polar). When thesurface of the second nitride crystal region 20 on the side opposite tothe third nitride crystal region 30 is a Group V surface (e.g., anitrogen surface), the polarity of the second nitride crystal region 20is Group V-polar (e.g., nitrogen-polar).

It was found that in the nitride crystal according to the embodiment,the crystal orientation of the second nitride crystal region 20 can beset to the reverse of the crystal orientation of the first nitridecrystal region 10 by providing a third nitride crystal region 30 such asthat described above. It was found that the polarity of the crystal ofthe second nitride crystal region 20 can be set to the reverse of thepolarity of the crystal of the first nitride crystal region 10 byproviding a third nitride crystal region 30 such as that describedabove.

For example, the nitride crystal according to the embodiment is obtainedby processing a layer used to form the first nitride crystal region 10in an atmosphere including oxygen, subsequently forming a layer used toform the third nitride crystal region 30 at a relatively low temperatureon the layer used to form the first nitride crystal region 10, andforming a layer used to form the second nitride crystal region 20 on thelayer used to form the third nitride crystal region 30. An example ofanalysis results of a sample of the nitride crystal 110 made by such amethod will now be described. An example of a method for manufacturingthe nitride crystal according to the embodiment is described below.

FIG. 3 is an electron microscope image illustrating the nitride crystalaccording to the first embodiment.

FIG. 3 is an example of a transmission electron microscope image of asample of the nitride crystal 110 according to the embodiment. In thissample, the first nitride crystal region 10 and the second nitridecrystal region 20 are GaN, and the third nitride crystal region 30 isAlN. As shown in FIG. 3 , the third nitride crystal region 30 is betweenthe first nitride crystal region 10 and the second nitride crystalregion 20.

FIGS. 4A to 4D are schematic views illustrating the nitride crystalaccording to the first embodiment. FIGS. 4A and 4C are angle-controlledannular bright field-scanning transmission electron microscopy(ABF-STEM) images of the sample of the nitride crystal 110 according tothe embodiment. FIG. 4A corresponds to the second nitride crystal region20. FIG. 4C corresponds to the first nitride crystal region 10. FIG. 4Bschematically shows the order of the atoms of the second nitride crystalregion 20. FIG. 4D schematically shows the order of the atoms of thefirst nitride crystal region 10.

As shown in FIGS. 4A to 4D, the crystal orientation of the secondnitride crystal region 20 is the reverse of the crystal orientation ofthe first nitride crystal region 10.

The crystal orientation of the first nitride crystal region 10 and thecrystal orientation of the second nitride crystal region 20 of thenitride crystal 111 according to the embodiment are respectively thereverse of those of the nitride crystal 110.

FIGS. 5A and 5B are graphs illustrating the nitride crystal according tothe first embodiment.

FIGS. 5A and 5B illustrate SIMS (Secondary Ion Mass Spectrometry)results of the sample of the nitride crystal 110 according to theembodiment. In these figures, the horizontal axis is a position pZ inthe Z-axis direction. The vertical axis of FIG. 5A is the concentrationCO of oxygen (O) or the concentration CO of carbon (C). The verticalaxis on the left side of FIG. 5B is the concentration CO of silicon(Si). The vertical axis on the right side of FIG. 5B is an intensityInt1 of the detection of aluminum (Al).

As shown in FIG. 5A, the concentration CO of oxygen (a third oxygenconcentration CO3) in the third nitride crystal region 30 is greaterthan the concentration CO of oxygen (a first oxygen concentration CO1)in the first nitride crystal region 10. The third oxygen concentrationCO3 is greater than the concentration CO of oxygen (a second oxygenconcentration CO2) in the second nitride crystal region 20.

As shown in FIG. 5A, the concentration CO of carbon (a third carbonconcentration CC3) in the third nitride crystal region 30 is greaterthan the concentration CO of carbon (a first carbon concentration CC1)in the first nitride crystal region 10. The third carbon concentrationCC3 is greater than the concentration CO of carbon (a second carbonconcentration CC2) in the second nitride crystal region 20.

In the embodiment, for example, the first oxygen concentration CO1 isnot more than 1/1000 of the third oxygen concentration CO3. For example,the second oxygen concentration CO2 is not more than 1/1000 of the thirdoxygen concentration CO3.

In the embodiment, for example, the first carbon concentration CC1 isnot more than 1/100 of the third carbon concentration CC3. For example,the second carbon concentration CC2 is not more than 1/100 of the thirdcarbon concentration CC3.

For example, the third oxygen concentration CO3 is 1×10²⁰/cm³ or more.The third oxygen concentration CO3 may be 1×10²¹/cm³ or more. When thethird oxygen concentration CO3 is 1×10²⁰/cm³ or more, the crystalorientation of the second nitride crystal region 20 easily becomes thereverse of the crystal orientation of the first nitride crystal region10. It is more favorable for the third oxygen concentration CO3 to be1×10²¹/cm³ or more. When the third oxygen concentration CO3 is1×10²¹/cm³ or more, for example, the uniformity of the crystalorientation of the second nitride crystal region 20 is easily increased.

For example, it is favorable for the third oxygen concentration CO3 tobe 1×10²³/cm³ or less. When the third oxygen concentration CO3 isgreater than 1×10²³/cm³, there are cases where the uniformity of thecrystal orientation of the second nitride crystal region 20 decreases.When the third oxygen concentration CO3 is greater than 1×10²³/cm³, forexample, there are cases where the quality of the crystal (e.g., thecrystallinity) of the second nitride crystal region 20 decreases.

For example, the third carbon concentration CC3 is 5×10¹⁸/cm³ or more.The third carbon concentration CC3 may be 1×10¹⁹/cm³ or more. When thethird carbon concentration CC3 is 5×10¹⁸/cm³ or more, the crystalorientation of the second nitride crystal region 20 easily becomes thereverse of the crystal orientation of the first nitride crystal region10. It is more favorable for the third carbon concentration CC3 to be1×10¹⁹/cm³ or more. When the third carbon concentration CC3 is1×10¹⁹/cm³ or more, for example, the uniformity of the crystalorientation of the second nitride crystal region 20 is easily increased.

For example, it is favorable for the third carbon concentration CC3 tobe 1×10²¹/cm³ or less. When the third carbon concentration CC3 isgreater than 1×10²¹/cm³, there are cases where defects (e.g.,dislocations) occur at the interface between the third nitride crystalregion 30 and the second nitride crystal region 20. When the thirdcarbon concentration CC3 is greater than 1×10²¹/cm³, there are caseswhere the quality of the crystal (e.g., the crystallinity) of the secondnitride crystal region 20 decreases.

The crystal orientation can be inverted by providing the third nitridecrystal region 30 in which the concentration CO of oxygen and theconcentration CO of carbon are high. For example, bonds of Al—C—O—Al,O—Al—O, etc., are formed in the third nitride crystal region 30. It isconsidered that the orientation of the crystal is inverted thereby. Forexample, bonds of Ga—C—O—Ga, O—Ga—O, etc., are formed in the thirdnitride crystal region 30. It is considered that the orientation of thecrystal is inverted thereby.

In the example as shown in FIG. 5A, the first oxygen concentration CO1is greater than the second oxygen concentration CO2. The first carbonconcentration CC1 is less than the second carbon concentration CC2. Forsuch a concentration relationship, for example, the <0001> direction ofthe first nitride crystal region 10 easily becomes the first orientation(e.g., the −Z direction). For example, the <0001> direction of thesecond nitride crystal region 20 easily becomes the second orientation(e.g., the +Z direction).

In the embodiment, the first oxygen concentration CO1 may be less thanthe second oxygen concentration CO2, and the first carbon concentrationCC1 may be greater than the second carbon concentration CC2. For such aconcentration relationship, for example, the <0001> direction of thefirst nitride crystal region 10 easily becomes the second orientation(e.g., the +Z direction). For example, the <0001> direction of thesecond nitride crystal region 20 easily becomes the first orientation(e.g., the −Z direction).

In the example as shown in FIG. 5B, the Al composition ratios in thefirst and second nitride crystal regions 10 and 20 are less than the Alcomposition ratio in the third nitride crystal region 30. For example,the position where the concentration of Al becomes ⅓ of the peak valuemay be taken as the boundary between the first nitride crystal region 10and the third nitride crystal region 30 or the boundary between thesecond nitride crystal region 20 and the third nitride crystal region30.

As shown in FIG. 5B, the third nitride crystal region 30 may includesilicon (Si). For example, the first nitride crystal region 10 and thesecond nitride crystal region 20 do not include silicon. Or, theconcentrations CO of silicon in the first and second nitride crystalregions 10 and 20 are less than the concentration CO of silicon in thethird nitride crystal region 30. For example, in SIMS, the state of theconcentration CO of silicon being not more than the detection lowerlimit (e.g., not more than background level) practically may correspondto the “state of not including silicon”.

Because the third nitride crystal region 30 includes silicon, thedislocation density in the layers (the second nitride crystal region 20,etc.) provided on the third nitride crystal region 30 can be low.

For example, it is favorable for the concentration CO of silicon in thethird nitride crystal region 30 to be not less than 1×10¹⁸/cm³ and notmore than 1×10²⁰/cm³. For example, the dislocation density can beeffectively reduced by setting the concentration CO of silicon in thethird nitride crystal region 30 to be 1×10¹⁸/cm³ or more. An unnecessaryconductivity does not occur easily when the concentration CO of siliconin the third nitride crystal region 30 is 1×10²⁰/cm³ or less.

It is favorable for the Mg concentration in the third nitride crystalregion 30 to be less than 1×10¹⁶/cm³. For example, an unnecessaryconductivity does not occur easily thereby.

For example, the front surface of the nitride semiconductor can have twotypes of polarities. One polarity is Group III-polar. The other polarityis Group V-polar (nitrogen-polar). The nitride semiconductor hasdifferent characteristics according to the polarity. It is consideredthat the features of the two types of polarities can be practically usedin a stacked structure in which nitride semiconductors include the twotypes of polarities (Group III-polar and Group V-polar(nitrogen-polar)). For example, it is easier to provide higherfunctionality or higher performance of the nitride semiconductor.

For example, the In incorporation efficiency for the nitrogen polarityis greater than the In incorporation efficiency for the Group IIIpolarity. By appropriately controlling the polarity, for example, ahigh-quality InGaN layer with a high In-composition-ratio can be formed.

In the embodiment, the polarity can be changed by providing a thirdnitride crystal region 30 such as that described above. According to theembodiment, a nitride crystal can be provided in which thecharacteristics can be improved.

In the embodiment, the third nitride crystal region 30 includes, forexample, Al_(x3)Ga_(1-x3)N (0<x3≤1). The third nitride crystal region 30may include, for example, AlN.

In the embodiment, the first nitride crystal region 10 includesAl_(x1)Ga_(1-x1)N (0≤x1≤1). The second nitride crystal region 20includes Al_(x2)Ga_(1-x2)N (0≤x2≤1).

In one example, for example, the third nitride crystal region 30 mayinclude AlN, and the first nitride crystal region 10 and the secondnitride crystal region 20 may include AlN. In one example, for example,the third nitride crystal region 30 may include AlN, and the firstnitride crystal region 10 and the second nitride crystal region 20 mayinclude AlGaN. In one example, for example, the third nitride crystalregion 30 may include AlN, and the first nitride crystal region 10 andthe second nitride crystal region 20 may include GaN.

For example, the third nitride crystal region 30 includesAl_(x3)Ga_(1-x3)N (0.9≤x3≤1). In such a case, the first nitride crystalregion 10 includes, for example, Al_(x1)Ga_(1-x1)N (0≤x1<0.9). In such acase, the second nitride crystal region 20 includes, for example,Al_(x2)Ga_(1-x2)N (0≤x2<0.9).

As shown in FIG. 1 or FIG. 2 , the third nitride crystal region 30 has athickness t30. The thickness t30 is, for example, the length along theZ-axis direction. The thickness t30 is, for example, the length alongthe <0001> direction.

In the embodiment, it is favorable for the thickness t30 to be, forexample, 6 nm or more. When the thickness t30 is 6 nm or more, forexample, the crystal orientation of the second nitride crystal region 20easily becomes the reverse of the crystal orientation of the firstnitride crystal region 10. The thickness t30 may be 10 nm or more. It ismore favorable for the thickness t30 to be 10 nm or more. For example,the uniformity of the crystal orientation of the second nitride crystalregion 20 is easily increased thereby.

It is favorable for the thickness t30 to be, for example, 70 nm or less.When the thickness t30 is much greater than 70 nm, for example, thereare cases where a part of the orientation of the crystal in the thirdnitride crystal region 30 again inverts, and the crystal orientation ofthe second nitride crystal region 20 becomes the same as the crystalorientation of the first nitride crystal region 10. It is more favorablefor the thickness t30 to be, for example, 50 nm or less. When thethickness t30 is greater than 50 nm, for example, there are cases wheredefects (e.g., dislocations) easily occur at the interface between thethird nitride crystal region 30 and the second nitride crystal region20. When the thickness t30 is greater than 50 nm, for example, there arecases where the quality of the crystal (e.g., the crystallinity) of thesecond nitride crystal region 20 easily decreases.

FIG. 6 is a schematic cross-sectional view illustrating a nitridecrystal according to the first embodiment.

As shown in FIG. 6 , the nitride crystal 112 according to the embodimentincludes a nitride semiconductor layer 50L in addition to the firstnitride crystal region 10, the second nitride crystal region 20, and thethird nitride crystal region 30. For example, the nitride semiconductorlayer 50L is provided on the second nitride crystal region 20. Thenitride semiconductor layer 50L may be, for example, a functional layer.According to the nitride crystal 112 as well, a nitride crystal can beprovided in which the characteristics can be improved.

The nitride crystal 112 may be included in a semiconductor device 210according to the embodiment. According to the semiconductor device 210,a semiconductor device can be provided in which the characteristics canbe improved.

An example of a method for manufacturing the nitride crystal 112according to the embodiment will now be described.

FIGS. 7A to 7E are schematic cross-sectional views illustrating themethod for manufacturing the nitride crystal according to the firstembodiment.

As shown in FIG. 7A, the nitride member 55 is provided on the substrate50 s. The nitride member 55 is, for example, a buffer layer. A firstnitride crystal layer 10L that is used to form the first nitride crystalregion 10 is provided on the nitride member 55. The first nitridecrystal layer 10L may be formed by epitaxial growth. In one example, thefirst nitride crystal layer 10L is GaN. For example, the upper surfaceof the first nitride crystal layer 10L (the surface on the side oppositeto the substrate 50 s) is a Group V surface (e.g., a nitrogen surface).For example, the upper surface of the first nitride crystal layer 10L iscaused to be a Group V surface (e.g., a nitrogen surface) by nitridingthe front surface of the substrate 50 s. For example, the nitride member55 is formed on the substrate 50 s by processing the substrate 50 s inan atmosphere including a Group V source gas and by subsequentlyperforming growth using a Group V source gas and a Group III source gas.The Group V source gas includes, for example, ammonia. The Group IIIsource gas includes, for example, TMGa (trimethylgallium).

For example, the upper surface of the first nitride crystal layer 10L(the surface on the side opposite to the substrate 50 s) becomes a GroupIII surface because the atoms of the front surface of the substrate 50 sare Group III atoms. For example, the nitride member 55 is formed byprocessing the substrate 50 s in an atmosphere including a Group IIIsource gas and by subsequently performing growth using a Group V sourcegas and a Group III source gas. A case where the upper surface of thefirst nitride crystal layer 10L is a Group V surface (e.g., a nitrogensurface) will now be described.

The first nitride crystal layer 10L illustrated in FIG. 7A is processedin an atmosphere including oxygen. Thereby, for example, as shown inFIG. 7B, oxygen 10X adsorbs to the front surface (the upper surface) ofthe first nitride crystal layer 10L. The processing in the atmosphereincluding oxygen includes, for example, processing in an ozoneatmosphere. The processing in the atmosphere including oxygen mayinclude, for example, placing a structure body including the firstnitride crystal layer 10L in air. The processing in the atmosphereincluding oxygen may include, for example, heat treatment in anatmosphere including oxygen. In such a case, the temperature of thesubstrate 50 s is, for example, not less than 200° C. and not more than400° C.

As shown in FIG. 7C, a third nitride crystal layer 30L is formed on thefirst nitride crystal layer 10L after the processing described above. Inone example, the third nitride crystal layer 30L is AlN. For example,the third nitride crystal layer 30L is formed using a Group V source gasand a Group III source gas. The Group V source gas includes, forexample, ammonia. The Group III source gas includes, for example, TMAI(trimethylaluminum).

The temperature is 450° C. or less in the formation of the third nitridecrystal layer 30L. For example, the temperature of the substrate 50 s isset to be not less than 350° C. and not more than 450° C. By forming thethird nitride crystal layer 30L at such a low temperature, carbon iseasily incorporated into the third nitride crystal layer 30L.

The Group V/Group III ratio in the formation of the third nitridecrystal layer 30L is, for example, 20000 or more. For example, the GroupV/Group III ratio is not less than 20000 and not more than 100000. Bysuch a Group V/Group III ratio, oxygen is easily incorporated into thethird nitride crystal layer 30L. The polarity of the crystal changes(e.g., inverts) in the third nitride crystal layer 30L.

As shown in FIG. 7D, a second nitride crystal layer 20L is formed on thethird nitride crystal layer 30L. In one example, the second nitridecrystal layer 20L is GaN. The crystal orientation of the second nitridecrystal layer 20L is inverted with respect to the crystal orientation ofthe first nitride crystal layer 10L. For example, the upper surface(e.g., the front surface) of the second nitride crystal layer 20L is aGroup III surface (e.g., a Ga surface).

As shown in FIG. 7E, the nitride semiconductor layer 50L also may beformed on the second nitride crystal layer 20L. The nitridesemiconductor layer 50L may be, for example, a functional layer. Thepolarity of the nitride semiconductor layer 50L is the same as thepolarity of the second nitride crystal layer 20L.

At least a portion of the first nitride crystal layer 10L is used toform the first nitride crystal region 10. At least a portion of thethird nitride crystal layer 30L is used to form the third nitridecrystal region 30. At least a portion of the second nitride crystallayer 20L is used to form the second nitride crystal region 20.

The first nitride crystal layer 10L described above may be formed asfollows. For example, the substrate 50 s (e.g., a sapphire substrate) isintroduced to the processing chamber of a MOCVD apparatus. Thetemperature of the substrate 50 s is increased. Heat treatment isperformed in a hydrogen atmosphere in a state in which the temperatureof the substrate 50 s is 1100° C. For example, thermal cleaning isperformed. Subsequently, ammonia is introduced to the processingchamber, and the front surface of the substrate 50 s is nitrided in astate in which the temperature of the substrate 50 s is 1100° C.

Subsequently, the temperature of the substrate 50 s is set to 470° C.,and a GaN layer (the nitride member 55) is formed in a hydrogenatmosphere by supplying TMGa (trimethylgallium) and ammonia. The nitridemember 55 is a buffer layer. The thickness of the nitride member 55 is,for example, about 7 nm.

Subsequently, the temperature of the substrate 50 s is set to 1050° C.,and a GaN layer (the first nitride crystal layer 10L) is formed in ahydrogen atmosphere by supplying TMGa and ammonia. The thickness of thefirst nitride crystal layer 10L is, for example, 300 nm.

Subsequently, the temperature of the substrate 50 s is lowered to roomtemperature. Excimer light is irradiated on the structure body includingthe first nitride crystal layer 10L in an oxygen atmosphere. Thereby,oxygen is adsorbed to the front surface of the first nitride crystallayer 10L.

Subsequently, the temperature of the substrate 50 s is increased to 400°C. in a nitrogen atmosphere. In this state, AlN (the third nitridecrystal layer 30L) is formed by supplying TMAI and ammonia. At thistime, the Group V/Group III ratio is, for example, 18000.

Subsequently, the temperature of the substrate 50 s is set to 1100° C.,and TMGa and ammonia are supplied in a hydrogen atmosphere. A GaN layer(the second nitride crystal layer 20L) is formed thereby. The thicknessof the second nitride crystal layer 20L is, for example, about 300 nm.

FIG. 8 is a schematic cross-sectional view illustrating a nitridecrystal according to the first embodiment.

In the nitride crystal 113 according to the embodiment as shown in FIG.8 , the third nitride crystal region 30 includes multiple regions.Otherwise, the nitride crystal 113 may be similar to the nitride crystal112.

As shown in FIG. 8 , the third nitride crystal region 30 includes afirst partial region 31, a second partial region 32, and a third partialregion 33. The first partial region 31 includes Al_(y1)Ga_(1-y1)N(0<y1≤1). The second partial region 32 is provided between the firstpartial region 31 and the second nitride crystal region 20. The secondpartial region 32 includes Al_(y2)Ga_(1-y2)N (0<y2≤1). The third partialregion 33 is provided between the first partial region 31 and the secondpartial region 32. The third partial region 33 includesAl_(y3)Ga_(1-y3)N (0≤y3<1, y3<y1, and y3<y2).

In one example, for example, the first partial region 31 and the secondpartial region 32 include AlN, and the third partial region 33 includesGaN. In such a third nitride crystal region 30 as well, the polarity canbe changed (e.g., inverted). The dislocation density can be reduced bythe third nitride crystal region 30 that includes regions havingdifferent composition ratios of Al.

In one example, the first partial region 31 and the second partialregion 32 include AlN, and the third partial region 33 includes AlGaN.In such a third nitride crystal region 30 as well, the polarity can bechanged (e.g., inverted). For example, the dislocation density can bereduced by the third nitride crystal region 30 including regions havingdifferent composition ratios of Al.

For example, in the nitride crystal 112 described above (referring toFIG. 6 ), the Al composition ratio in the third nitride crystal region30 is substantially constant. The threading dislocation density in thenitride crystal 112 is about 1.2×10⁸/cm².

On the other hand, in the nitride crystal 113 described above, the thirdnitride crystal region 30 includes multiple regions havingmutually-different composition ratios of Al. The threading dislocationdensity in the nitride crystal 113 is about 6.0×10⁷/cm².

The nitride crystal 113 may be included in a semiconductor device 211according to the embodiment. According to the semiconductor device 211,a semiconductor device can be provided in which the characteristics canbe improved.

In another example according to the embodiment, the first nitridecrystal region 10 is AlN, the third nitride crystal region 30 is AlN,and the second nitride crystal region 20 is AlN. The upper surface ofthe first nitride crystal region 10 (the surface on the third nitridecrystal region 30 side) is a nitrogen surface. The upper surface of thesecond nitride crystal region 20 (the surface on the third nitridecrystal region 30 side) is an Al surface. In such a case, the polarityof the first nitride crystal region 10 is nitrogen-polar. The polarityof the second nitride crystal region 20 is Group III-polar (Al-polar).

In another example according to the embodiment, the first nitridecrystal region 10 is GaN, the third nitride crystal region 30 is AlN,and the second nitride crystal region 20 is GaN. The upper surface ofthe first nitride crystal region 10 (the surface on the third nitridecrystal region 30 side) is a Ga surface. The upper surface of the secondnitride crystal region 20 is a nitrogen surface. In such a case, thepolarity of the first nitride crystal region 10 is Group III-polar(Ga-polar). The polarity of the second nitride crystal region 20 isnitrogen-polar.

In another example according to the embodiment, the first nitridecrystal region 10 is AlN, the third nitride crystal region 30 is AlN,and the second nitride crystal region 20 is AlN. The upper surface ofthe first nitride crystal region 10 (the surface on the third nitridecrystal region 30 side) is an Al surface. The upper surface of thesecond nitride crystal region 20 (the surface on the third nitridecrystal region 30 side) is a nitrogen surface. In such a case, thepolarity of the first nitride crystal region 10 is Group III-polar(Al-polar). The polarity of the second nitride crystal region 20 isnitrogen-polar.

In another example according to the embodiment, the first nitridecrystal region 10 is GaN, the third nitride crystal region 30 isAl_(0.2)Ga_(0.8)N, and the second nitride crystal region 20 is GaN. Theupper surface of the first nitride crystal region 10 (the surface on thethird nitride crystal region 30 side) is a nitrogen surface. The uppersurface of the second nitride crystal region 20 (the surface on the sideopposite to the third nitride crystal region 30) is a Ga surface. Insuch a case, the polarity of the first nitride crystal region 10 isnitrogen-polar. The polarity of the second nitride crystal region 20 isGroup III-polar (Ga-polar).

In another example according to the embodiment, the first nitridecrystal region 10 is GaN, the third nitride crystal region 30 isAl_(0.2)Ga_(0.8)N, and the second nitride crystal region 20 is GaN. Theupper surface of the first nitride crystal region 10 (the surface on thethird nitride crystal region 30 side) is a Ga surface. The upper surfaceof the second nitride crystal region 20 (the surface on the sideopposite to the third nitride crystal region 30) is a nitrogen surface.In such a case, the polarity of the first nitride crystal region 10 isGroup III-polar (Ga-polar). The polarity of the second nitride crystalregion 20 is nitrogen-polar.

Second Embodiment

FIG. 9 is a schematic cross-sectional view illustrating a semiconductordevice according to a second embodiment.

As shown in FIG. 9 , the semiconductor device 220 according to theembodiment includes the nitride semiconductor layer 50L and the nitridecrystal according to the first embodiment. The nitride semiconductorlayer 50L is provided on the nitride crystal. In the example, thenitride crystal 112 is used as the nitride crystal described above.

In the example, the nitride semiconductor layer 50L includes a firstregion 51 and a second region 52. The first region 51 is between thenitride crystal 112 and the second region 52. The first region 51includes Al_(z1)Ga_(1-z1)N (0≤z1<1). The second region 52 includesAl_(z2)Ga_(1-z2)N (z1<z2≤1).

In the example, the semiconductor device 220 includes a first electrode71, a second electrode 72, a third electrode 73, and an insulatingmember 75. In the example, the position in the X-axis direction of thethird electrode 73 is between the position in the X-axis direction ofthe first electrode 71 and the position in the X-axis direction of thesecond electrode 72. The first electrode 71 is electrically connected toa portion of the nitride semiconductor layer 50L. The second electrode72 is electrically connected to another portion of the nitridesemiconductor layer 50L. The insulating member 75 is provided betweenthe third electrode 73 and the nitride semiconductor layer 50L. Acurrent that flows between the first electrode 71 and the secondelectrode 72 can be controlled by a potential of the third electrode 73.The semiconductor device 220 is, for example, a HEMT (High ElectronMobility Transistor).

The semiconductor device 220 includes the nitride crystal 112 thatincludes inverted polarities. In the semiconductor device 220, forexample, the upper surface (the surface on the side opposite to thesubstrate 50 s) of the region proximate to the substrate 50 s is anitrogen surface. For example, the <0001> direction of the regionproximate to the substrate 50 s is the −Z direction. For example, a lowdislocation density is obtained thereby. The upper surface (the surfaceon the side opposite to the substrate 50 s) of the region distant to thesubstrate 50 s is, for example, a Group III surface (e.g., a Gasurface). For example, the <0001> direction of the region distant to thesubstrate 50 s is the +Z direction. For example, high mobility isobtained thereby. For example, high reliability is easily obtained.According to the semiconductor device 220, a semiconductor device can beprovided in which the characteristics can be improved.

FIG. 10 is a schematic cross-sectional view illustrating a semiconductordevice according to the second embodiment.

As shown in FIG. 10 , the semiconductor device 221 according to theembodiment includes the nitride semiconductor layer 50L and the nitridecrystal 112 according to the first embodiment. In the example, thenitride semiconductor layer 50L includes the first region 51, the secondregion 52, and a third region 53.

The first region 51 is between the nitride crystal 112 and the secondregion 52. The third region 53 is between the first region 51 and thesecond region 52. The first region 51 is of a first conductivity type(e.g., an n-type). The second region 52 is of a second conductivity type(e.g., a p-type).

The third region 53 includes at least one of a higher In concentrationthan the In concentration in the first region 51 or a lowerconcentration of Al than the Al concentration in the first region 51.The third region 53 includes at least one of a higher In concentrationthan the In concentration in the second region 52 or a lowerconcentration of Al than the Al concentration in the second region 52.The third region 53 includes, for example, a well layer. The thirdregion 53 is, for example, a light-emitting region. The first electrode71 is electrically connected to the first region 51. The secondelectrode 72 is electrically connected to the second region 52. Thesemiconductor device 221 is, for example, a light-emitting device.

The semiconductor device 221 includes the nitride crystal 112 thatincludes inverted polarities. In the semiconductor device 221, forexample, the upper surface (the surface on the side opposite to thesubstrate 50 s) of the region proximate to the substrate 50 s is anitrogen surface. For example, the <0001> direction of the regionproximate to the substrate 50 s is the −Z direction. In thesemiconductor device 221, for example, a low dislocation density isobtained. The upper surface (the surface on the side opposite to thesubstrate 50 s) of the region distant to the substrate 50 s is, forexample, a Group III surface (e.g., a Ga surface). For example, the<0001> direction of the region distant to the substrate 50 s is the +Zdirection. In the semiconductor device 221, for example, a high luminousefficiency is obtained.

Third Embodiment

FIGS. 11A and 11B are schematic views illustrating an optical deviceaccording to a third embodiment.

FIG. 11A is a perspective view. FIG. 11B is a cross-sectional view.

As shown in FIGS. 11A and 11B, the optical device 310 according to theembodiment includes the nitride crystal (e.g., the nitride crystal 110)according to the first embodiment. For example, a first light L1 isincident on the side surface of the nitride crystal 110. A second lightL2 is emitted from another side surface of the nitride crystal 110. Inthe nitride crystal 110, the polarity of the second nitride crystalregion 20 is the reverse of the polarity of the first nitride crystalregion 10. Thereby, the second light L2 is the second harmonic of thefirst light L1. The wavelength of the second light L2 is ½ of thewavelength of the first light L1. For example, the optical device 310functions as a wavelength conversion device.

For example, the wavelength of the first light L1 is 810 nm. Thewavelength of the second light L2 is 405 nm. In the optical device 310according to the embodiment, a wavelength conversion device is obtainedby a simple configuration. According to the embodiment, an opticaldevice can be provided in which the characteristics can be improved.

Fourth Embodiment

FIG. 12 is a flowchart illustrating a method for manufacturing a nitridecrystal according to a fourth embodiment.

As shown in FIG. 12 , the manufacturing method according to theembodiment includes processing the first nitride crystal layer 10L in anatmosphere including oxygen (step S110). After the processing, themanufacturing method includes forming the third nitride crystal layer30L on the first nitride crystal layer 10L with a Group V/Group IIIratio of 20000 or more at a temperature of 450° C. or less (step S120).The manufacturing method includes forming the second nitride crystallayer 20L on the third nitride crystal layer 30L (step S130). Forexample, the processing described with reference to FIGS. 7B to 7D isperformed. According to the embodiment, a method for manufacturing anitride crystal can be provided in which the characteristics can beimproved.

In the embodiment, for example, the third oxygen concentration CO3 inthe third nitride crystal layer 30L is greater than the first oxygenconcentration CO1 in the first nitride crystal layer 10L and greaterthan the second oxygen concentration CO2 in the second nitride crystallayer 20L. For example, the third carbon concentration CC3 in the thirdnitride crystal layer 30L is greater than the first carbon concentrationCC1 in the first nitride crystal layer 10L and greater than the secondcarbon concentration CC2 in the second nitride crystal layer 20L.

In the embodiment, the substrate 50 s may be, for example, a siliconsubstrate. The substrate 50 s may be, for example, a sapphire substrate.The substrate 50 s may be, for example, a SiC substrate. The substrate50 s may be, for example, a GaN substrate. The substrate 50 s may be,for example, an AlN substrate.

In the embodiment, the nitride member 55 may include, for example, anAlN. The nitride member 55 may include, for example, a stacked body inwhich multiple AlGaN layers are stacked. The nitride member 55 may have,for example, a superlattice structure in which GaN and AlN arealternately stacked.

In the embodiments, “nitride semiconductor” includes all compositions ofsemiconductors of the chemical formula B_(x)In_(y)Al_(z)Ga_(1-x-y-z)N(0≤x≤1, 0≤y≤1, 0≤z≤1, and x+y+z≤1) for which the composition ratios x,y, and z are changed within the ranges respectively. “Nitridesemiconductor” further includes Group V elements other than N (nitrogen)in the chemical formula recited above, various elements added to controlvarious properties such as the conductivity type and the like, andvarious elements included unintentionally.

According to the embodiments, a nitride crystal, an optical device, asemiconductor device, and a method for manufacturing a nitride crystalcan be provided in which the characteristics can be improved.

Hereinabove, exemplary embodiments of the invention are described withreference to specific examples. However, the embodiments of theinvention are not limited to these specific examples. For example, oneskilled in the art may similarly practice the invention by appropriatelyselecting specific configurations of components included insemiconductor devices such as conductive portions, semiconductorregions, insulating portions, interconnects, etc., from known art. Suchpractice is included in the scope of the invention to the extent thatsimilar effects thereto are obtained.

Further, any two or more components of the specific examples may becombined within the extent of technical feasibility and are included inthe scope of the invention to the extent that the purport of theinvention is included.

Moreover, all nitride crystals, optical devices, semiconductor devices,and methods for manufacturing nitride crystals practicable by anappropriate design modification by one skilled in the art based on thenitride crystals, the optical devices, the semiconductor devices, andthe methods for manufacturing nitride crystals described above asembodiments of the invention also are within the scope of the inventionto the extent that the purport of the invention is included.

Various other variations and modifications can be conceived by thoseskilled in the art within the spirit of the invention, and it isunderstood that such variations and modifications are also encompassedwithin the scope of the invention.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the invention.

What is claimed is:
 1. A nitride crystal, comprising: a first nitridecrystal region; a second nitride crystal region; and a third nitridecrystal region provided between the first nitride crystal region and thesecond nitride crystal region, the third nitride crystal regionincluding Al, a third oxygen concentration in the third nitride crystalregion being greater than a first oxygen concentration in the firstnitride crystal region and greater than a second oxygen concentration inthe second nitride crystal region, a third carbon concentration in thethird nitride crystal region being greater than a first carbonconcentration in the first nitride crystal region and greater than asecond carbon concentration in the second nitride crystal region, a<0001> direction of the first nitride crystal region being one of afirst orientation from the second nitride crystal region toward thefirst nitride crystal region or a second orientation from the firstnitride crystal region toward the second nitride crystal region, and a<0001> direction of the second nitride crystal region being the other ofthe first orientation or the second orientation, wherein the firstoxygen concentration is greater than the second oxygen concentration,and the first carbon concentration is less than the second carbonconcentration.
 2. The nitride crystal according to claim 1, wherein thefirst oxygen concentration is not more than 1/1000 of the third oxygenconcentration, the second oxygen concentration is not more than 1/1000of the third oxygen concentration, the first carbon concentration is notmore than 1/100 of the third carbon concentration, and the second carbonconcentration is not more than 1/100 of the third carbon concentration.3. The nitride crystal according to claim 1, wherein the third oxygenconcentration is 1×10²⁰/cm³ or more, and the third carbon concentrationis 5×10¹⁸/cm³ or more.
 4. The nitride crystal according to claim 1,wherein the <0001> direction of the first nitride crystal region is thefirst orientation, and the <0001> direction of the second nitridecrystal region is the second orientation.
 5. The nitride crystalaccording to claim 1, wherein the third nitride crystal region includessilicon, and the first nitride crystal region and the second nitridecrystal region do not include silicon, or concentrations of silicon inthe first nitride crystal region and the second nitride crystal regionare less than a concentration of silicon in the third nitride crystalregion.
 6. The nitride crystal according to claim 5, wherein theconcentration of silicon in the third nitride crystal region is not lessthan 1×10¹⁸/cm³ and not more than 1×10²⁰/cm³.
 7. The nitride crystalaccording to claim 1, wherein a concentration of Mg in the third nitridecrystal region is less than 1×10¹⁶/cm³.
 8. The nitride crystal accordingto claim 1, wherein the third nitride crystal region includesAl_(x3)Ga_(1-x3)N (0<x3≤1).
 9. The nitride crystal according to claim 8,wherein the first nitride crystal region includes Al_(x1)Ga_(1-x1)N(0≤x1≤1), and the second nitride crystal region includesAl_(x2)Ga_(1-x2)N (0≤x2≤1).
 10. The nitride crystal according to claim1, wherein the third nitride crystal region includes Al_(x3)Ga_(1-x3)N(0.9≤x3≤1), the first nitride crystal region includes Al_(x1)Ga_(1-x1)N(0≤x1<0.9), and the second nitride crystal region includesAl_(x2)Ga_(1-x2)N (0≤x2<0.9).
 11. The nitride crystal according to claim1, wherein the third nitride crystal region includes: a first partialregion including Al_(y1)Ga_(1-y1)N (0<y≤1); a second partial regionprovided between the first partial region and the second nitride crystalregion, the second partial region including Al_(y2)Ga_(1-y2)N (0<y2≤1);and a third partial region provided between the first partial region andthe second partial region, the third partial region includingAl_(y3)Ga_(1-y3)N (0≤y3<1, y3<y1, and y3<y2).
 12. The nitride crystalaccording to claim 1, further comprising: a substrate, the first nitridecrystal region being between the substrate and the second nitridecrystal region.
 13. The nitride crystal according to claim 1, wherein athickness of the third nitride crystal region is not less than 6 nm andnot more than 70 nm.
 14. An optical device, comprising: the nitridecrystal according to claim
 1. 15. A semiconductor device, comprising:the nitride crystal according to claim 1; and a nitride semiconductorlayer.
 16. The semiconductor device according to claim 15, wherein thenitride semiconductor layer includes a first region and a second region,the first region is between the nitride crystal and the second region,the first region includes Al_(z1)Ga_(1-z1)N (0≤z1<1), and the secondregion includes Al_(z2)Ga_(1-z2)N (z1<z2≤1).
 17. The semiconductordevice according to claim 15, wherein the nitride semiconductor layerincludes a first region, a second region, and a third region, the firstregion is between the nitride crystal and the second region, the thirdregion is between the first region and the second region, the firstregion is of a first conductivity type, the second region is of a secondconductivity type, and the third region includes at least one of ahigher In concentration than an In concentration in the first region ora lower concentration of Al than a concentration of Al in the firstregion.
 18. A nitride crystal, comprising: a first nitride crystalregion; a second nitride crystal region; and a third nitride crystalregion provided between the first nitride crystal region and the secondnitride crystal region, the third nitride crystal region including Al, athird oxygen concentration in the third nitride crystal region beinggreater than a first oxygen concentration in the first nitride crystalregion and greater than a second oxygen concentration in the secondnitride crystal region, a third carbon concentration in the thirdnitride crystal region being greater than a first carbon concentrationin the first nitride crystal region and greater than a second carbonconcentration in the second nitride crystal region, a <0001> directionof the first nitride crystal region being one of a first orientationfrom the second nitride crystal region toward the first nitride crystalregion or a second orientation from the first nitride crystal regiontoward the second nitride crystal region, and a <0001> direction of thesecond nitride crystal region being the other of the first orientationor the second orientation, wherein the third nitride crystal regionincludes Al_(x3)Ga_(1-x3)N (0.9≤x3≤1), the first nitride crystal regionincludes Al_(x1)Ga_(1-x1)N (0≤x1<0.9), and the second nitride crystalregion includes Al_(x2)Ga_(1-x2)N (0≤x2<0.9).